Wireless temperature calibration device and method

ABSTRACT

A method for calibrating at least one temperature sensor. A wafer (30) having calibration structures of a material having a melting point in the range of 150° to 1150° C. is provided. The temperature sensor is operable to detect a temperature dependent characteristic of the wafer and output a signal corresponding to the temperature depending characteristic. The power input is selectively varied and the wafer temperature is ramped for a calibration run. A wafer characteristic, such as wafer reflectance, radiance, or emissivity, is monitored. A first step change in the wafer characteristic corresponding to a wafer temperature equal to the melting point of the calibration structures is detected and a set of calibration parameters for each temperature sensor being calibrated is calculated.

This is a continuation, division, of application Ser. No. 07/928,564,filed Aug. 11, 1992, now U.S. Pat. No. 5,265,957 issued Nov. 30, 1993.

NOTICE

Copyright, *M* Texas Instruments Incorporated 1992. A portion of thedisclosure of this patent document contains material which is subject tocopyright and mask work protection. The copyright and mask work ownerhas no objection to the facsimile reproduction by anyone of the patentdocument or the patent disclosure, as it appears in the Patent andTrademark Office patent file or records, but otherwise reserves allcopyright and mask work rights whatsoever.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to sensor technology and morespecifically to wireless temperature calibration devices and methods forsemiconductor device thermal fabrication processes.

2. Description of the Related Art

Numerous semiconductor device thermal fabrication processes employtemperature sensors which require calibration against reliable andrepeatable standards. An important group of device fabrication processesis rapid thermal processing (RTP). Most of the RTP reactors employnon-contact pyrometry for wafer temperature measurement and control, asshown in FIG. 1. However, the accuracy and repeatability of RTPtemperature measurement by pyrometry depend strongly on the waferemissivity. In practice, frequent pyrometry sensor calibrations arerequired in order to obtain acceptable process repeatability. Thesefrequent cross-calibrations are usually performed by placing separatestandard calibration wafers with bonded thermocouples (TC-bonded wafers)in the process chamber, as shown in FIG. 1. The TC-bonded calibrationwafers are placed in the process chamber between a quartz window and agas showerhead. However, the TC-bonded calibration wafers requireexternal electrical connections to the TC wires. As a result, thecalibration process needs manual loading and unloading of the TC-bondedwafer. This manual process is time consuming and is not suitable in asemiconductor device manufacturing environment due to its detrimentalimpact on equipment utilization. This problem is even more severe whenTC-bonded wafers with multiple distributed thermocouples are requiredfor calibration of multi-point pyrometry sensors housed in a multi-zoneilluminator. In addition, thermocouple-assisted temperature calibrationsare not suitable in reactive (e.g. oxidizing) ambients at highertemperatures (>950° C). This is due to the fact that the thermocouplejunctions degrade rapidly under these conditions. Even under inertconditions, thermocouples have limited lifetime. Thus, TC-bonded wafersexhibit limited lifetime and can only be used for a limited number ofmanual calibration runs. TC-bonded wafers may also introduce calibrationerrors (of as much as 10° C. or larger) due to the localized temperatureoffset caused by thermal loading. Special bonding procedures arerequired to minimize the sources of calibration error. CommercialTC-Bonded wafers are available for temperature sensor calibrations;these TC-bonded wafers are, however, expensive. Finally, the manualthermocouple-assisted calibrations may introduce contaminants into theprocess chamber. This can reduce the device manufacturing yield,particularly in critical processes such as gate dielectric formation andepitaxial silicon growth.

OBJECTS OF THE INVENTION

It is therefore an object of the invention to provide a wirelesscalibration system.

Another object of the invention is providing a wireless calibrationsystem having known precise and repeatable temperature calibrationpoints based on constant physical parameters such that it requires noinitial calibration.

A further object of the invention is providing a wireless calibrationsystem that operates in automatic wafer handling process without anyneed for manual handling.

A further object of the invention is providing a wireless calibrationsystem that operates in both inert and reactive ambients.

A further object of the invention is providing a wireless calibrationsystem with an increased life span which can be used repeatedly forprecise and repeatable temperature sensor calibrations.

SUMMARY OF THE INVENTION

Precise and repeatable calibration of wafer temperature sensors, such aspyrometry, has been a problem. Generally, and in one form of theinvention, a device and method for calibrating at least one temperaturesensor is described. A wafer is provided which has a first plurality ofcalibration islands. The islands are of a material which has a meltingpoint in the range 150°-1150° C. The wireless device is operable toinduce a step change in an output of each temperature sensor at a wafertemperature equal to said melting point during a ramped-temperaturecalibration process.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a block diagram of a prior art TC-bonded wafer calibrationsystem shown in conjunction with a multi-zone rapid thermal processingsystem.

FIG. 2 is a cross-sectional diagram of a temperature calibration waferaccording to the first preferred embodiment of the present invention;

FIG. 3a-d are cross-sectional diagrams of a temperature calibrationwafer according to the first preferred embodiment of the presentinvention in various states of device fabrication;

FIG. 4 is a top view of a temperature calibration device with twodifferent, calibration elements according to the second preferredembodiment of the present invention.

FIG. 5 is a cross-sectional diagram of a temperature calibration devicewith two different calibration elements according to the secondpreferred embodiment of the present invention;

FIG. 6a-c are cross-sectional diagrams of a temperature sensorcalibration wafer according to the second preferred embodiment of thepresent invention in various states of fabrication;

FIG. 7 is a graph of spectral radiance output of the temperature sensorcalibration wafers versus time during a slow temperature/power ramp in athermal processing chamber;

FIG. 8 is a block diagram of a rapid thermal processor with a wirelesstemperature calibration system according to the invention;

FIG. 9 is a qualitative graph of both the output radiance of thetemperature sensor calibration wafer versus time and the calibrationwafer reflectance values versus time during a slow temperature/powerramp; and

FIG. 10 is a top view of a wireless temperature calibration waferaccording to a third preferred embodiment of the invention.

FIGS. 11a-b are cross-sectional diagrams of the fourth preferredembodiment of the invention.

Corresponding numerals and symbols in the different figures refer tocorresponding parts unless otherwise indicated.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The preferred embodiments of the invention will be described withreference to a rapid thermal processing (RTP) reactor using pyrometrysensors for wafer temperature measurement, such as that shown in FIG. 1.The invention may alternatively be used with other device fabricationequipment and temperature measurement techniques, such as thermalexpansion-based temperature sensors, acoustical sensors, andellipsometry-based sensors. Typical processes which require precisetemperature measurement and control include chemical-vapor deposition(CVD), thermal oxidation, and thermal anneals. However, the invention isnot limited in application to semiconductor device fabricationprocesses. For example, it may also be used in conjunction with flatpanel display (FPD) fabrication processes.

In contrast to prior art devices, the calibration wafers of the presentinvention have no thermocouples (TC's) and do not require any initialcalibration against TC-bonded wafers before their use for precisepyrometry calibrations (or calibrations of other temperature sensors).The calibration wafer of this invention offers known precise temperaturecalibration standard points based on constant physical parameters.Because there are no thermocouples, the calibration wafers are wireless.Thus, they allow automated wafer handling for the calibration process.No manual wafer handling inside an automated processing reactor isneeded. This feature makes the calibration standard of this inventioncompatible with product manufacturing environments. Furthermore, thecalibration wafers of this invention can be used repeatedly for numeroustemperature calibrations both in inert and reactive (e.g. oxidizing andnitridizing) ambients. These wafers are expected to last for many morecalibration runs compared to the conventional TC-bonded wafers.

The invention can be implemented both on dedicated standard calibrationwafers or on actual device wafers. For the latter, the temperaturecalibration elements are placed on the wafer backsides so that eachdevice wafer serves as its own calibration standard as well. In theformer, the calibration elements may be placed either on the waferfrontside (polished side) or on its backside.

This invention allows placement of one or two (or even multiple)calibration element types on each wafer. Each calibration element typeis associated with a single known precise temperature. Two calibrationtemperature points (T₁ and T₂) are sufficient in order to calibratepyrometry sensors in their operating temperature range for both gain andoffset as will be discussed below. In this invention, melting points ofsuitable elements are used as the calibration temperature points. Themelting points of pure elements and alloys of two (or more) elementswith known ratios are physical constants which can be used for sensorcalibration purposes when solid-to-melt phase transitions occur during athermal cycle. These phase transitions can be detected in real time bynoncontact means as will now be explained.

It is a known that various material elements demonstrate abrupt changesin their physical characteristics such as optical reflectance during thesolid-to-melt phase transition at the melting point. For instance,surface optical reflectance of germanium (Ge) shows a step change(increase) when a transition from solid to melt occurs at the meltingpoint (Tm=937.4° for Ge). Other material properties such as electricalresistivity and microwave reflectance may also show abrupt step changesat the material melting point. The preferred material property used forthe purpose of this invention is the optical reflectance or emissivityof the wafer at the melting points of the materials used and the abruptchanges associated with it.

A cross-section of the first preferred embodiment of the invention isshown in FIG. 2. Wafer 30 contains a buffer layer structure 32 locatedadjacent substrate 28 either on the frontside or the backside of wafer30 (frontside is the preferred choice). Islands 36 of calibrationmaterial (e.g. germanium) are fabricated adjacent buffer layer structure32. Buffer layer structure 32 may, for example, consist of both an oxidelayer 33 and a nitride layer 34 to prevent the calibration material ofislands 36 from reacting with the substrate during thermal calibrationruns. Encapsulation layer 38 covers and isolates calibration islands 36.Since islands 36 will melt and solidify during calibration,encapsulation layer 38 seals each island 36 to contain the calibrationelements 36. Finally, passivation layer 40 (such as silicon nitride) islocated adjacent encapsulation layer 38. Passivation layer 40 preventsdegradation of islands 36 in reactive environments such as oxygen.

The process for forming the first preferred embodiment will now bedescribed with reference to FIGS. 3a through d. As shown in FIG. 3a,initial buffer barrier layers 33, 34 are deposited over the substrate 28of wafer 30. The buffer structure may consist of an initial layer ofsilicon dioxide 33 and a top layer of silicon nitride 34. Silicondioxide layer 33 may, for example, have thickness of approximately 1000Å and may be formed by low pressure chemical-vapor deposition (LPCVD).As will be apparent to those skilled in the art, other methods such asplasma-enhanced chemical-vapor deposition (PECVD), or thermal oxidationmay alternatively be used. Silicon nitride layer 34 may also have athickness of approximately 1000 Å and may be formed by LPCVD. Again, aswill be apparent to those skilled in the art, other methods such asPECVD may alternatively be used. The buffer structure may also be madeof other suitable material layers such as refractory metals.

Next, as shown in FIG. 3b, a thin layer of the desired calibrationmaterial 35, such as germanium (Ge) is deposited. The melting point,T_(m1), for Ge is 937.2° C. Deposition may be accomplished by varioustechniques such as CVD or physical-vapor deposition (PVD) such assputtering. Calibration layer 35 has a thickness in the range of 200 and3000 Å and is typically around 2000 Å. Calibration materials are chosenby three criteria. First, the melting points must be in the temperaturerange of interest. Typically, this is between 150° and 1150° C. Second,high boiling points and low vapor pressures are required to preventcontamination and stress-induced peeling of the encapsulation layers andto allow numerous calibration runs. Finally, the calibration materialmust comprise suitable elements or alloys which are compatible withsilicon processing technology to prevent reactor contamination. Table 1shows some examples of preferred materials for the purpose of thisinvention.

                  TABLE 1                                                         ______________________________________                                                    Melting   Boiling Solid  Melting                                              Temp.     Temp.   Density                                                                              Density                                  Element/Alloy                                                                             (°C.)                                                                            (°C.)                                                                          (g/cm.sup.3)                                                                         (g/cm.sup.3)                             ______________________________________                                        Aluminum (Al)                                                                             660.37    2467    2.6989 2.370                                    Bismuth (Bi)                                                                              271.3     1560           10.05                                    Germanium (Ge)                                                                            937.4     2830    5.323  5.57                                     Indium (In) 156.61    2080    7.31   7.01                                     Tin (Sn)    231.97    2270    7.31   6.98                                     10% Al + 90% Sn                                                                           540.0                                                             ______________________________________                                    

Calibration layer 35 is then patterned via microlithography and plasmaetch (or wet etch) to forman array of Ge islands 36, as shown in FIG.3c. It is preferred that the islands 36 cover all of the wafer surfaceas shown in FIG. 4. However, they may alternatively only cover a portionof the wafer surface. The typical dimensions of the Ge islands are 25μm×25 μm (gaps of 2.5 μm between adjacent islands). Larger or smallerdimensions may also be used. FIG. 4 shows islands 36 as square, butother shapes such as hexagons may of course alternatively be used. TheGe patterning etch may be performed in chlorine-containing (e.g. Cl₂) orfluorine-containing (e.g. SF₆) plasmas.

An encapsulation layer 38 of SiO₂ (or silicon nitride) is deposited viaLPCVD or PECVD, as shown in FIG. 3d. Those skilled in the art willrecognize that other methods such as sputtering may also be used. Atypical encapsulation layer thickness is 1000 Å.

Finally, the passivation layer 40 is deposited. Passivation layer 40 mayconsist silicon nitride approximately 1000 Å thick and may, for example,be deposited by PECVD. Passivation layer 40 will prevent oxidation ofthe calibration elements during calibration runs in reactive oxidizingambients. The resultant structure is shown in FIG. 2.

A cross-section of the second preferred embodiment of the invention isshown in FIG. 5. Wafer 30 contains a stacked buffer layer 32 (or asingle buffer layer) located adjacent substrate 28 and may be on eitherthe frontside or the backside of wafer 30 (frontside placement ispreferred). Islands of a first calibration material 36 are locatedadjacent buffer layer 32. Buffer layer 32 may, for example, consists ofboth an oxide layer and a nitride layer to prevent the calibrationmaterial of islands 36 from reacting with the substrate during thermalcalibration. Encapsulation layer 38 covers and isolates islands 36.Islands of second calibration material 46 are located aboveencapsulation layer 38. Encapsulation layer, 48 seals the islands ofsecond calibration material 46. Finally, passivation layer 40 is locatedadjacent encapsulation layer 48. Passivation layer 40 (silicon nitride)prevents degradation of islands 36 and 46 in reactive environments suchas oxidation. A single layer of silicon nitride may be used for bothencapsulation and passivation.

The process for forming the second preferred embodiment will now bedescribed with reference to FIG. 6a through c. FIG. 6a is across-sectional diagram of wafer 30 having buffer layers 33, and 34,first calibration islands 36 and encapsulation layer 38. These layersare formed in the same manner as described with respect to the firstpreferred embodiment.

As shown in FIG. 6b, a thin layer (e.g., 1000 Å) of second calibrationmaterial 45 is deposited above encapsulation layer 38. This may beaccomplished using CVD or PVD for example. Second calibration materialmay for example consist of tin. Preferred choices for first and secondcalibration material include germanium and tin, germanium and aluminum,or aluminum and tin, for example. For tin and germanium (Sn, Ge), Tm₁ isapproximately 237° C. and T_(m2) is approximately 937° C. Thiscombination is good for sensor calibrations over an extended temperaturerange without contamination problems (e.g. for applications such asrapid thermal oxidation, rapid thermal anneal, etc.). Both germanium andtin are column IV semiconductors and are not considered contaminants insilicon. Aluminum and germanium is a preferred combination whenT_(m1) >400° is required for pyrometry signal calibrations. This is dueto the fact that some pyrometry sensors do not provide sufficient signallevels for temperatures less than 400° C. Tin and aluminum provide aT_(m1) approximately equal to 232° C. and T.sub. m2 approximately 660°C. This combination is useful for wireless calibrations inlow-temperature processes. Examples include silicide react/annealprocesses which are in the temperature range of 550°-750° C.

After the second calibration material is deposited, a second patterningstep is performed to forman array of second calibration material islands46, as shown in FIG. 6c. Islands 46 may have the same dimension as firstislands 36. The pattern of islands 46 is such that it will not causeshadowing of the first calibration elements. This will ensure that theaverage local reflectance/emissivity of the wafer at any point isdetermined by the reflectivities of both calibration elements.

Finally, as shown in FIG. 5, a second encapsulation layer 50 and thepassivation layer 40 are deposited. Second encapsulation layer 50 mayconsist of SiO₂ at a thickness of approximately 1000 Å. Passivationlayer 40 may consist of silicon nitride approximately 1000 Å thick. Bothlayers may be deposited by, for example, PECVD. Passivation layer 40will prevent oxidation of the calibration elements during calibrationruns in oxidizing ambients. A single top layer of silicon nitride may beused for both encapsulation and passivation purposes.

The first and second preferred embodiments can be used for wirelesstemperature calibration runs for both single point and multi-pointpyrometry sensors of a RTP reactor. In operation, the calibration pointsT_(m1) and T_(m2) can be detected by one of several methods. Thepreferred method of detecting T_(m1) and T_(m2) calibration points isdirectly via the pyrometry signal or signals. As the wafer temperatureis raised (or ramped) above T_(m2) (or T_(m1)), a step change in waferreflectance will result in a step change in its effective emissivity andthe resultant pyrometry signal(s) will also experience a small stepchange, as shown in FIG. 7. Assuming a slow temperature/power ramp(change or temperature variation e.g., during an open-loop powerramp-up):

    T=∝I+β                                         (1)

Where,

I is the linealized pyrometer output current or voltage;

T is the calibrated wafer temperature;

α is the slope or linear coefficient; and

β is the offset.

If one calibration material is used, as in the first preferredembodiment, T_(m1) and I₁ are determined. Since the slope will generallybe known from a separate thermocouple calibration run, the offset can becalculated as follows:

    β=T.sub.m1 -∝I.sub.1                           (2)

If two or more calibration materials are used, both the slope and offsetcan be calculated because T_(m1) and I₁ as well as T_(m2) and I₂ will beknown.

    T.sub.m1 =∝I.sub.1 +β                          (3)

    T.sub.m2 =∝I.sub.2 +β                          (4)

    T.sub.m2 -T.sub.m1 =∝(I.sub.2 -I.sub.1)             (5) ##EQU1##

The wireless calibration wafer usually has the same backside emissivityas the actual device wafers (although the requirement is not critical ifthe pyrometry system also employs emissivity compensation). In amulti-point pyrometry sensor system, the same wireless calibration wafercan be used for simultaneous calibrations of all the pyrometry sensors Adistinct (I_(i1), T_(m1)) and (I_(i2), T_(m2)) will be determined foreach sensor and a separate slope (gain) and offset can then becalculated for each sensor.

An alternative method to detect T_(m1) and T_(m2) transitions forpyrometry calibrations (rather than trying to detect the small pyrometrysignal step changes) is to use laser beams 118 from the frontside (oreven backside) to monitor the surface reflectivity values, as shown inFIG. 8. A laser beam 118 is directed at wafer 112 across from eachsensor 1-4. Sensors 1-4 are located within the multi-zone illuminator ofthe RTP reactor and separated from wafer 112 by quartz window 110. Onthe other side of wafer 112 is gas showerhead 114. In the preferredembodiment, sensors 1-4 are pyrometers. However, it will be apparent tothose skilled in the art that sensors 1-4 may alternatively beelipsometers, thermal expansion sensors, or acoustical sensors. Thelaser may be a cheap HeNe (6328 Å) laser with fiber-optic coupling 120for improved alignment. The reflectance values R₁ -R₄ are monitoredduring a calibration cycle power ramp using detectors 116. The surfacereflectance values measured during a calibration cycle will experiencestep changes at times corresponding to (I_(i1), T_(m1)) and (I_(i2),T_(m2)), as shown in FIG. 9. Accordingly, T_(m1) and I₁ and T_(m2) andI₂ are determined and the above calculations for slope (gain) and offsetapply.

A third preferred embodiment of the invention is shown in FIG. 10. Thisfigure shows formation of the calibration elements on narrow rings 60 onthe backside of wafer 62. The rings may have a typical width of 2 mm orless and are located at radial positions corresponding to the radialprobe positions of the multi-point pyrometry sensors. The rings mayconsist of one or more calibration materials. Any of the two detectionmethods for (Ii₁, T_(m1)) and (I_(i2), T_(m2)), direct pyrometry stepdetection or laser reflectance, can be used. In this case, the HeNelaser probes can be placed into the illuminator light pipes to look atthe specific radial positions on the wafer backside (similar structureas the pyrometer light pipes).

Referring to FIGS. 11a-b, a fourth preferred embodiment involves the useof two separate wireless calibration wafers. Both wafers 30A and 30B maybe formed as described above relative to the first preferred embodimentand shown in FIG. 2. Calibration islands 36A and 36B are located withinencapsulation layers 38A and 38B, respectively. Encapsulation layers 38Aand 38B are separated from substrate 28A and 28B by buffer layer 32A and32B, respectively. Passivation layers 40A and 40B cover wafers 30A and30B respectively. However, each wafer will have a different calibrationmaterial. In operation, Ii₁ and T_(m1) are determined using the firstwafer by either of the two detection method described above. I_(i2) andT_(m2) are determined using the second wafer. Once (Ii₁, T_(m1)) and(I_(i2), T_(m2)) are known, the slope and offset can be calculatedaccording to equations (6) and (7).

A few preferred embodiments have been described in detail hereinabove.It is to be understood that the scope of the invention also comprehendsembodiments different from those described, yet within the scope of theclaims.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. It is therefore intended that the appended claims encompassany such modifications or embodiments.

What is claimed is:
 1. A method for calibrating at least one temperaturesensor comprising the steps of:providing a wafer having at least oneplurality of calibration structures of a material having a melting pointin the range of 150° to 1150° C., wherein said at least one temperaturesensor is operable to detect a temperature dependent characteristic ofsaid wafer and output a signal corresponding to said temperaturedepending characteristic; selectively varying a power input and rampingthe wafer temperature to said temperature depending characteristic;selectively varying a power input and ramping the wafer temperature fora calibration run; directing at least one light beam at the wafer;monitoring the wafer surface reflectance at the point at which eachlight beam is directed during the step of ramping the wafer temperature;detecting a first step change in the wafer surface reflectancecorresponding to a wafer temperature equal to the melting point of saidcalibration structures; determining a first set of sensor parameterscorresponding to said first step change; and calculating a set ofcalibration parameters for each temperature sensor being calibrated. 2.The method of claim 1 wherein said wafer has a first and secondplurality of calibration structures, each of said pluralities comprisinga different material having a melting point in the range 150° to 1150°C.
 3. The method of claim 2 further comprising the step of detecting asecond step change in the wafer surface reflectance corresponding to awafer temperature equal to the melting point of said second calibrationstructure and determining a second set of sensor parameterscorresponding to said second step change.
 4. The method of claim 1,wherein said first set of sensor parameters comprises current levels. 5.The method of claim 1 wherein said at least one temperature sensorcomprises a pyrometer.
 6. The method of claim 1 wherein said temperaturedependent characteristic is wafer radiance.
 7. The method of claim 1wherein said temperature dependent characteristic is wafer emissivity.8. A method for calibrating at least one temperature sensor comprisingthe steps of:providing a wafer having a first plurality of calibrationislands of a material having a melting point in the range 150° to 1150°C.; selectively varying a power input and ramping the temperature ofsaid wafer; monitoring a wafer emissivity of the wafer with said atleast one temperature sensor while said wafer temperature is beingramped; detecting a first step change in said wafer emissivitycorresponding to a wafer temperature equal to said melting point of saidfirst calibration islands for each temperature sensor being calibrated;calculating the calibration parameters for each temperature sensor beingcalibrated.
 9. The method of claim 8 wherein said wafer also has aplurality of second calibration islands of a material different fromsaid first calibration islands and having a melting point in the range150° to 1150° C.
 10. The method of claim 9 further comprising the stepof detecting a second step change in said wafer emissivity correspondingto a wafer temperature equal to the melting point of said secondcalibration material for each temperature sensor being calibrated. 11.The method of claim 8, further comprising the steps of:providing asecond wafer having a second plurality of calibration islands of asecond material having a melting point in the range 150° to 1150° C.,wherein said second material is different from said first material;slowly ramping the temperature of said second wafer; monitoring saidwafer emissivity of said second wafer in response to said increasingtemperature for each temperature sensor; and detecting a second stepchange in said wafer emissivity corresponding to a second wafertemperature equal to said melting point of said second calibrationislands for each temperature sensor.
 12. The method of claim 11 furthercomprising the steps of:directing at least one light beam at said wafer;and providing a detector for each light beam to detect waferreflectivity, wherein said first and second step changes are detected inan output signal of said detector.
 13. A method for calibrating at leastone temperature sensor comprising the steps of:providing a wafer havinga first plurality of calibration islands of a material having a meltingpoint in the range 150° to 1150° C.; selectively varying a power inputand ramping the temperature of said wafer; monitoring a wafer radianceof the wafer with said at least one temperature sensor while said wafertemperature is being ramped; detecting a first step change in said waferradiance corresponding to a wafer temperature equal to said meltingpoint of said first calibration islands for each temperature sensorbeing calibrated; calculating the calibration parameters for eachtemperature sensor being calibrated.
 14. The method of claim 13 whereinsaid wafer also has a plurality of second calibration islands of amaterial different from said first calibration islands and having amelting point in the range 150° to 1150° C.
 15. The method of claim 14further comprising the step of detecting a second step change in saidwafer radiance corresponding to a wafer temperature equal to the meltingpoint of said second calibration material for each temperature sensorbeing calibrated.
 16. The method of claim 13, further comprising thesteps of:providing a second wafer having a second plurality ofcalibration islands of a second material having a melting point in therange 150° to 1150° C., wherein said second material is different fromsaid first material; slowly ramping the temperature of said secondwafer; monitoring said wafer radiance of said second wafer in responseto said increasing temperature for each temperature sensor; anddetecting a second step change in said wafer radiance corresponding to asecond wafer temperature equal to said melting point of said secondcalibration islands for each temperature sensor.
 17. The method of claim16 further comprising the steps of:directing at least one light beam atsaid wafer; and providing a detector for each light beam to detect waferreflectivity, wherein said first and second step changes are detected inan output signal of said detector.